1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, an electrophotographic printer and the like, and more particularly, it relates to an image forming apparatus using an intermediate transfer member.
2. Related Background Art
FIG. 11 is a schematic constructural view of a conventional image forming apparatus using an intermediate transfer member.
A photosensitive drum 101 rotated in a direction shown by the arrow R1 is uniformly charged by a charge roller 102. Then, scan exposure is effected by using a laser beam 103 ON/OFF-controlled in response to image information to form an electrostatic latent image on the photosensitive drum.
The electrostatic latent image is developed by a developing means 104. The developing means 104 includes a rotary 104A, and four developing devices mounted on the rotary. The developing devices are developing devices 104a, 104b, 104c and 104d in which yellow (first color) non-magnetic one-component toner, magenta (second color) non-magnetic one-component toner, cyan (third color) non-magnetic one-component toner, and black (fourth color) non-magnetic one component toner are contained, respectively. Each developing device has a developing roller for adhering the toner to the electrostatic latent image. By rotating the rotary 104A, the developing device selected to be used for the development of the electrostatic latent image on the photosensitive drum 101 is brought to a developing station where the developing device is opposed to the photosensitive drum 101. The above-mentioned electrostatic latent image is developed (visualized) by the developing device 104a with yellow toner to form an yellow (first) toner image.
The visualized first toner image is electrostatically transferred (firstly-transferred) onto a surface of an intermediate transfer belt 105 at a first toner station 106a where the toner image is opposed to the intermediate transfer belt 105 which is rotated in a direction shown by the arrow. After the first-transferring, a small amount of residual toner (first-transferring residual toner) remaining on the photosensitive drum 101 is removed by a cleaning device 107.
Then, the above-mentioned process is repeated regarding the remaining colors, i.e., magenta, cyan and black. In this way, a magenta (second) toner image, a cyan (third) toner image and a black (fourth) toner image are successively firstly-transferred onto the surface of the intermediate transfer belt 105 in a superimposed fashion.
Thereafter, a second transfer roller 108 which was disengaged from the surface of the intermediate transfer belt 105 (which roller can be engaged by and disengaged from the intermediate transfer belt along a direction shown by the arrow K8) is urged against the surface of the intermediate transfer belt 105 and is rotated, so that the toner images formed on the surface of the intermediate transfer belt 105 are collectively transferred (secondary-transferred) onto a surface of a transfer material P conveyed to a second transfer station 106b at a predetermined timing. Thereafter, the transfer material P is conveyed to a fixing device (not shown), where the toner images are fixed onto the transfer material as a permanent image. Then, the transfer material is discharged out of the apparatus.
After the secondary-transferring, a small amount of residual toner (secondary-transferring residual toner) remaining on the surface of the intermediate transfer belt 105 is removed by a cleaning roller 109 (which can be engaged by and disengaged from the intermediate transfer belt along a direction shown by the arrow K9) which is urged against the surface of the intermediate transfer belt 105 by a drive means (not shown) at a predetermined timing. The cleaning roller 109 is constituted by a core cylinder, and a conductive elastic layer coated on the core cylinder. While the toner images on the surface of the photosensitive drum 101 are being transferred onto the surface of the intermediate transfer belt 105 successively, the cleaning roller is spaced apart from the surface of the intermediate transfer belt 105, and, after the toner images on the surface of the intermediate transfer belt 105 are secondary-transferred onto the surface of the intermediate transfer belt 105 are secondary-transferred onto the surface of the transfer material P collectively, the cleaning roller is urged against (engaged by) the surface of the intermediate transfer belt 105 and bias is applied to the cleaning roller from a high voltage power source 110.
As a result, the secondary-transferring residual toner remaining on the surface of the intermediate transfer belt 105 is charged to polarity opposite to normal charging polarity of toner. At the same time, when a first color yellow toner image in a next image forming process is firstly-transferred from the surface of the photosensitive drum 101 onto the surface of the intermediate transfer belt 105, the secondary-transferring residual toner is reversely transferred from the surface of the intermediate transfer belt 105 onto the surface of the photosensitive drum 101 at the first transfer station 106a, thereby cleaning the surface of the intermediate transfer belt 105.
In the past, the entire longitudinal length (referred to as "effective charging area" hereinafter) of the charge roller 102 was selected to become slightly greater than a thin toner layer forming area on the surface of the developing sleeve. The reason is to prevent occurrence of the following phenomenon. That is to say, for example, in the development using a jumping developing method, since a small amount of toner is also scattered onto surface portions of the photosensitive drum outside of the thin toner layer forming area on the surface of the developing sleeve, if the charging potential of such surface portions is insufficient, the amount of the scattered toner is increased, thereby contaminating the interior of the apparatus. Further, it is practical that the cleaning roller 109 having the same configuration and construction as the charge roller 102 for charging the surface of the photosensitive drum 101 is used from the viewpoint of cost.
However, the image forming apparatus having the above-mentioned construction had the following problems.
That is to say, in order to effect the cleaning operation by using the cleaning roller 109, must be fully charged with polarity opposite to the polarity of the secondary-transferring residual toner charged by the cleaning roller 109 with polarity opposite to the normal polarity of toner. However, when the charge roller 102 and the cleaning roller 109 as described in connection with the above-mentioned conventional example are used, deformed amounts (compressed amounts) of the rollers are great at both longitudinal ends of the rollers, so that the adequate discharge area cannot be obtained. Consequently, charging ability becomes unstable, with the result that, for example, in the vicinity of ends of the areas of the surface of the intermediate transfer belt 105 against which the cleaning roller 109 is urged, the secondary-transferring residual toner which was not reversely transferred to the photosensitive drum 101 is accumulated, thereby contaminating the output print with toner.
Further, particularly in an image forming apparatus of the type in which a process cartridge integrally including the photosensitive drum and the charge roller is mounted to a main body of the apparatus having the intermediate transfer belt 105 or an intermediate transfer drum (not shown), the charge roller 102 of the process cartridge is offset from the cleaning roller 109 of the main body along the longitudinal direction. Thus, the surface of the photosensitive drum 101, the area of the surface of the photosensitive drum 101 charged by the charge roller 102 and the area of the intermediate transfer belt 105 charged by the cleaning roller 109 are not partially overlapped, thereby causing the above problem.